The proposed research is designed to investigate adult humans' difficulties in telling right from left as opposed to above from below. From everyday life, it seems clear that such a difficulty exists but the amount of research which has been done on this problem is surprisingly small. Reaction time experiments were designed to answer three questions. 1) Under what conditions are right-left judgments harder than above-below judgments? Pilot data have suggested that telling right from left is more difficult than telling above from below only when a person cannot devote all of his/her attention to the task. The size of the right-left effect will be observed under different levels of uncertainty and memory load. 2) What is the source of the diffuclty in telling right from left? Does the right-left difficulty arise in perception of the locations of two objects in space, in the translation of locational words into a representation of directionality, or bot? 3) Do ambidextrous and left-handed people have more difficulty in telling right from left than right-handed people? One hypothesis for the right-left difficulty involves the bilateral symmetry of the human nervous system. It has also been suggested that the nerous systems of right-handed people are less bilaterally symmetrical than those of non-right-handed individuals. Therefore, the literature suggests that right-handed people should experience less difficulty in telling right from left than should non-right-handed people. However, pilot data have suggested that just the opposite is the case and the effects of handedness on right-left judgments will be pursued in the proposed experiments.